A ligament is a band of connective tissue composed mainly of collagen fibres. The knee joint ligaments connect the femur (thighbone) to the tibia (leg bone) at the knee joint to improve its stability and to limit the amount of mobility in the joint.
The four main ligaments of the knee joint are,
- ACL: Anterior Cruciate Ligament
- PCL: Posterior Cruciate Ligament
- MCL: Medial Collateral Ligament
- LCL: Lateral Collateral Ligament
Functions of the knee ligaments
- Stability to the knee joint
The ligaments of the knee are responsible for preventing the tibia (shin bone) from sliding out of the femur (thigh bone). During rotational movements, knee ligaments work together to prevent both valgus (knee moved inwards) or varus (knee moved outwards) stresses to the knee.
According to their attachments in the knee, the ligaments prevent tibial displacements. For example, ACL prevents forward displacement of the tibia while PCL prevents backward displacement of the tibia. Similarily, the MCL provides support on the inner side of the knee while the LCL provides support on the outer side of the knee.
- Locking the knee during walking
Apart from supporting the bones, the knee ligaments contribute to the “screw-home” mechanism, a process that locks the knees during walking. For example, just before you strike the heel to the ground your knee is slightly flexed (about 20 degrees bent) then the screw home mechanism works to straighten the knee as your body moves over the planted heel as shown in Fig 2.
What does a ligament Injury mean?
A ligament injury is the over-stretching or tearing of the ligaments of the knee. A tear may be partial or complete.
What can cause a Knee Ligament Injury?
Extreme movements at the knee joint forcing the knee to move beyond its normal motion can injure a ligament. Most of the injuries occur during weight-bearing activities, as the ligaments resist against perturbations at the knee.
Types of people who usually get them
- Sports people like football players, basketball players, skiers etc.
- Hyper-mobile individuals who engage in high-impact sports may have an injury due to excessive laxity in the knee ligaments.
- Accidental fall on the knees or hit on the knees during contact sports like rugby, football etc or automobile accidents (in which the knees can hit the dashboard)
Mechanism of an injury
- Hyper extension injury
Extending the knee too far by over straightening of the knee. This can happen when you stop suddenly while running.
- Flexion and Hyperflexion injury
Jumping and landing on a flexed (bent) knee or falling on your knees with over overbent knees.
- Rotational injuries
Valgum (inner) or varum (outer) stress on the knees due to twisting of your knee inwards and outwards. Sudden shifting of weight from one leg to the other.
- Contact Sports
Accidental hit on the knee during sports as shown in Fig 5
Other Reasons that contribute to a Ligament Injury
- Lack of force distribution
During movement, the body exerts a force on the ground and at the same time, an equal and opposite ground reaction force (GRF) is exerted by the ground on the body. This GRF is directed towards the center of mass (COM) of the body, a point in the body where the entire body weight is concentrated; in front of the tailbone.
If there is an imbalance, which means the athlete’s knee does not bend on landing and remains straight, the GRF creates a forward shear force that pushes the tibial forwards, stressing the ligaments. Hamstring muscles on the back of the thigh play a vital role in stabilizing the knee joint especially when the athlete lands. Normally, the knees normally bend slightly to absorb GRF as shown in Fig 6.
- Lack of trunk control
Without trunk control, there will be greater movements in the trunk following a perturbation (disturbance) which could affect the distribution of the GRF.
Lack of control in the trunk motion happens because of diminished proprioception. In such a situation, if the trunk moves more on the side of the knee joint laterally, the GRF tracks the COG and follows the movement of the trunk. As the GRF tracks the COM, and if it progresses beyond the center of the knee joint, it results in a movement of the knee joint into a valgus alignment stressing the knee ligaments as shown in Fig 6.
Signs and Symptoms of Ligament Injury
- Popping sound at the time of injury can indicate a ligament rupture.
- The knee swelling within the first 24 to 48 hours
- Tenderness and possibly redness around your knee on touching.
- Knee feels unstable or may buckle during weight bearing. This may cause you to limp or feel wobbly at the knee during walking.
- Bruising around the knee can develop.
What to do if you think you have an injury?
If you are having any of the above signs or symptoms, seek immediate medical attention. What can appear to be a simple ligament or soft tissue strain may become something more if left untreated. Diagnostic tests such as an X-ray or MRI scan will be able to show any tears or rupture of the ligaments. According to the severity of the ligament injury, appropriate treatment care will be advised.